Diode laser-pumped neodymium lasers (Nd:YAG, for example) have proven a very useful source of green and blue laser radiation by frequency doubling with non-linear crystals. Other materials have also been used to develop solid-state lasers for use over other portions of the spectrum. For a review of the early state of the diode pumped laser art, reference is had to the article "Diode Laser-Pumped Solid-State Lasers" by Robert L. Byer, Science, Vol 239, pp 742-747 (1988).
Slab geometry has now been found preferable to more traditional rod geometry for the solid state component in diode pumped lasers, but, as power outputs have increased, effort have been required to control the heat generated in slab laser operation. While the geometry of the slab laser itself reduces the thermal problem to those in a single dimension, the remaining thermal requirements still represent a limitation in the power performance of diode-pumped slab lasers.
Slab lasers which are used to develop significant amounts of power are often configured for pumping through the side faces of the slab. During operation considerable heat is generated within the lasing slab which is, itself, a relatively poor thermal conductor. This condition, therefore, sets up the requirement that the slab be directly cooled by some means.
Current slab lasers which emit powers greater than several watts have required either direct liquid contact with the slab or intimate contact between the slab and a metal heat sink. Liquid coolants change the optical boundary conditions of the slab for total internal reflection (TIR), which is correctable, if inconvenient; however liquid may not touch the slab ends where it will cause losses and phase distortions. Therefore, a liquid seal is necessary around a rectangular slab. Also, sometimes the ions in the slab leach out into the cooling liquid, and the liquid itself must be pumped past the slab and through the cooling member. All of these requirements add mechanical complexity to the design.
Where metals have been brought into contact with the slab, their inherent roughness (aluminum, for example, cannot be made to the required smoothness) allows only imperfect contact, leaving a print of a portion of the surface in contact interspersed with irregular air pockets and therefore is a poor arrangement as a thermal conductor. The slab may not be put into contact with a material which either absorbs the optical radiation (most metals) or which violates the TIR condition at the location of the reflections from the top and bottom surfaces. Therefore the slab may not be contacted with a metal or silicon for that matter for heat removal.
The part of the optical wave which bounces at the surface extends beyond the surface boundary into the neighboring medium up to a distance determined by the dielectric properties of the slab and the neighboring medium. When the Nd-YAG slab is in contact with air, this evanescent wave extends into the air to a distance of approximately 1 micron at the laser wavelength. This sets the minimum gap thickness but does depend on the material properties and the optical wavelength.
There has also been proposed the use of a thin layer of PTFE fluoroplastic coated onto the slab or onto a metal support for being pressed into intimate contact against the slab. Due to limitations in mechanical tolerances, the thermal resistance is nonuniform and high, and any application of stresses to reduce the thermal resistance due to poor contact introduces loss due to depolarization and distortion into the circulating laser beam.
There is, therefore, a need for an improved cooling system and mounting for use in slab lasers.